The invention relates to a method of manufacturing an optical fibre, in which method a rod-shaped initial preform having a core and a cladding surrounding this core is manufactured, the refractive index of the cladding being lower than that of the core, after which the initial preform is stretched by creating a softening zone in the initial preform, which softening zone is moved longitudinally along the initial preform, after which at least one cladding tube is melted on the initial preform and the final preform thus obtained is drawn into an optical fibre.
The term optical fibre is to be understood to mean a fibre of glass or quartz glass such as used for, for example, telecommunication purposes.
Such a method is described in U.S. Pat. No. 4,578,101, in which an initial preform is used which is manufactured by the vapour phase axial deposition (VAD) method. However, the method according to the invention can also be applied using initial preforms which are manufactured by different methods which are known per se, for example by a method in which a tube is internally clad by means of chemical deposition from the vapour phase (CVD or MCVD) or by plasma-activated chemical deposition from the vapour phase (PCVD) after which the tube is collapsed to manufacture a solid initial preform. According to said United States Patent, a hydrogen-oxygen torch or a heating furnace is used to create a softening zone in the initial preform while the initial preform is being stretched. The stretching process and the control thereof are further described by D. H. Smithgall and R. E. Frazee in Journal of Lightwave Technology, LT-5 (12), on pages 1755-1762 (1987).
A disadvantage of the known method is that it is slow and it requires the temperature and the velocities to be accurately adjusted. The hydrogen-oxygen flame is comparatively wide and produces a softening zone having a length of 10 to 12 cm. The temperature in the softening zone generally ranges from 1800.degree. C. to 2000.degree. C. and may reach 2200.degree. C. A narrower and hotter flame cannot suitably be used due to the occurrence of burn-off losses caused by convective, turbulent gas streams around the softening zone. This leads to loss of material and may result in an irregularly shaped initial preform.